“Junk food” is difficult to defi

“Junk food” is difficult to define. White sugar is probably the nearest contender for the title. It contains plenty of calories for energy but not much else, and is often described as an “empty calorie food. Alcohol is also high in calories, but beer and wine contain some of the B vitamins and wine is a what is a good source of iron, so even a teetotaler could not describe all alcohol as useless, nutritionally speaking. Calories measure the energy we derive from the food we eat, and sugar and alcohol are sometimes described as having a high energy density. There is a limit to the amount of energy we need each day (2,000-2,200 calories is the average for women and 2,500-3,000 for men) and if we eat too much sugar and alcohol there is no appetite left for the vitamin-rich foods we need- fish, meat, fruit and vegetables. Buying vitamins can be predicted by psychological as well as nutritional motives and it is prudent to investigate why we think we need them and what benefits we expect from them before we rush off to the health shop to make our purchases.
Most of us buy vitamins for one of three reasons. Either we believe that they are prophylactic, that is they will ward of advancing ill, or they are therapeutic and will deal with the ills we have already, or finally we may believe they are wonder drugs and will lift us into a state of super health, with all its attendant delights. We are protected from some of these wild imaginings by the laws which control advertising but even without false promises we still believe that vitamins
will “do us good". Belief is a very potent state of mind and the power of the placebo pill is never underestimated in clinical trials used to test new drugs. A placebo is a harmless substance given to one group of patients in the trial and it is similar in taste and appearance to a new drug which is given to a second group of patients. Theoretically the drug should cure or relieve any symptoms and the placebo should have no effect. Often these trials produce surprising results and the placebo group recovers as well as the group taking the new drug. It is a “mind over matter” philosophy and for some of us it works. Vitamin pills can sometimes fall into this category.
Vitamins B and C cannot be retained in the body, so if we take more than we need of these, they are soon excreted in the urine. The possible exception here is the theory about the increased body “pool” of vitamin C, but even this is limited and is still largely unproven. Taking too much of the fat soluble vitamins can be. dangerous and vitamins A and D should never be taken indiscriminately. Vitamin E has not been found to have any toxic effect in large doses, but neither do there seem to be any noticeable benefits. This is an unexplored area in vitamin research and the only known advantages of vitamin E are confined to specialized medical cases.


examples of particular problems (Para.3)
【正确答案】：cases